Tap actions take #3

This Pull Request covers the introduction of configurable tap actions.
The Double-Tap action and the Three-Finger action can be set to any of these values:

* Nothing
* Show original Artwork (= default behavior for Double-Tap and currently built into Muzei)
* Next Artwork (sends the Next command to the ArtSource)
* View Artwork details (opens the provided view Intent from the ArtSource)

This pull request has been re-based to the latest changes